CODEWITHSUNDEEP
c++pointers
Sarwagya
Sarwagya

Reputation: 166

What does char* (int) mean in C++?

I have the following line of code in one of the projects I am working on:

char* i2txt(int);

And I don't exactly understand what it does? With my half-knowledge I was trying to make it work with floats, so I changed the int to a float, but that gives me an error.

char* i2txt(int);
/*vs*/
char* i2txt(float);

Error Message :

Error LNK2019 unresolved external symbol "char * __cdecl i2txt(float)" (?i2txt@@YAPADM@Z) referenced in function "public: char * __thiscall DapChannel::getDurationTxt(void)" (?getDurationTxt@DapChannel@@QAEPADXZ)  PhotoZ  DapChannel.obj  1

Upvotes: 4

Views: 683

Answers (4)

BattleTested_закалённый в бою
BattleTested_закалённый в бою

Reputation: 4111

Its just a function declaration that take and int argument and return char* :

// Declaration :
char* i2txt(int);

// Definition :
char* i2txt(int n)
{
    // do something
}

But the occurred error is because there's an implementation of i2txt function that take an int argument and when you trying to change the declaration (especially if the implementation defined in an static library) it give you the linker error like the bellow :

error LNK2019: unresolved external symbol "char * __cdecl a(float)" (?a@@YAPADM@Z) referenced in...

In the normal state, if you call i2txt with a float value, it may cast float number to int, BUT if you trying to change the declaration of i2txt the link error will be occur (If the definition is in a static library).

Upvotes: 3

Alecto
Alecto

Reputation: 10740

The statement char* i2txt(int); is forward-declaring a function i2txt that takes an int as input, and returns a char*.

What is forward-declaration?

If you have a function used before it's declared, that results in an error:

#include <iostream>

int main() {
    foo(); // Error: foo not defined
} 
void foo() {
    std::cout << "Hello, world!"; 
}

Forward-declaration basically states "This function isn't defined yet, but I promise I'll define it eventually. In the above case, it'd look like this:

#include <iostream>

void foo(); // Forward declaration

int main() {
    foo(); // Now we can use it
}

void foo() {
    std::cout << "Hello, world!";
}

Why do you get an error when you change it to i2txt(float);?

This results in an error because suddenly, there's no i2txt(int) function to call. Because ints can be implicitly converted to float, the compiler still allows other functions to call i2txt(float), but no definition for i2txt(float) is ever provided, so there's a linker error:

#include <iostream>
char* i2txt(float);

int main() {
    std::cout << i2txt(10); // Tries calling i2txt(float)
}

// This provides a definition for i2txt(int), but the linker is still missing a definition for i2txt(float)
char* i2txt(int) {
    // ... stuff
}

Upvotes: 3

Martin Bonner supports Monica
Martin Bonner supports Monica

Reputation: 28987

It declares a function which takes an int (by value), and returns a point to char. Given the name (integer to text), the pointer is almost certainly to a null terminated sequence of characters which are the text version of the number. The pointer will either be to a static variable in the function (which means it is not thread safe, and the resulting text must be copied if you want to save it), or to a dynamically allocated array (in which case it must be freed). A function returning a std::string would be much better.

Upvotes: 3

Wutz
Wutz

Reputation: 2942

This declares a function accepting an integer and returning a char pointer.

I'm guessing the error you got after changing it was a linker error, telling you that it doesn't find a definition for i2txt with a float parameter. That's because the definition of this function, which is provided elsewhere, accepts an integer parameter not a float.

Upvotes: 3

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